Input stage apparatus and method having a variable reference voltage

ABSTRACT

Input stage having increased input signal noise margin and method for generating an output signal having a predetermined logic level based on the voltage level of an input signal. The input stage includes an input buffer generating an output signal having a logic level based on the voltage of the input signal relative to the voltage of the reference voltage signal. A voltage generator provides a variable output voltage signal that is used as the reference voltage by the input buffer. The voltage of the output voltage signal provided by the voltage generator is dependent on the logic value of the output signal of the input buffer. In this manner, the reference voltage applied to the input buffer can be adjusted based on the logic level of the outputs signal in order to provide increased input signal noise margin.

TECHNICAL FIELD

[0001] The present invention relates generally to input stagesgenerating an output signal having a predetermined logic level based onthe voltage level of an input signal, and in particular, input stageshaving increased input signal noise margin.

BACKGROUND OF THE INVENTION

[0002] Integrated circuits typically include a number of input/outputpins which are used for communication with additional circuitry. Forexample, an integrated memory device such as a dynamic random accessmemory (DRAM) includes both control inputs for receiving memoryoperation control signals, and data pins for bidirectional datacommunication with an external system or processor. Since theinformation provided to an integrated circuit through its pins includeboth control signals and data, it is important that the signals areaccurately received and interpreted by the integrated circuit for properoperation.

[0003] In addition to the need to maintain the accuracy and integrity ofsignals provided to an integrated circuit, as integrated circuits havebecome smaller, and the demand for power efficient integrated circuitsincreases, the voltage levels of the input and output signals havecontinued to be reduced. Input and output signals having lower voltageswings have the benefit of lower switching times and lower powerconsumption. New circuitry and methods have been developed toaccommodate the lower voltage levels while ensuring the accuracy andintegrity of the data provided by the signals. For example, input stagesgenerally provide an output signal having a defined logic level based onthe voltage level of an input signal. Input stages have traditionallyset the voltage levels at which the input signal causes either a logicHIGH or LOW output signal to be generated by designing input transistorshaving the appropriate sizes. However, to accommodate the reducedvoltage levels of the input signals, alternative methods and input stagedesigns have been developed.

[0004] One such input stage includes an input buffer that generates anoutput signal having a logic level based on the voltage level of aninput signal relative to a reference voltage VREF. That is, where theinput signal has a voltage level greater than the VREF voltage, theinput buffer generates a HIGH output signal, and where the voltage levelis less than the VREF voltage, a LOW output signal is generated.However, an issue with input signal noise margin may arise with theseconventional input stages. As illustrated in FIG. 1, where a constantreference voltage VREF is applied, it is possible to have inadvertentswitching of an input buffer due to a noisy system bus. The referencevoltage VREF is maintained at a steady voltage level VREFSSTL. When theIN signal crosses VREFSSTL due to noise, the input buffer switches thelogic level of the output signal OUT. Thus, although the IN signal isintended to transition once from a relatively low voltage level to arelatively high voltage level at a time t_(H), and then once again backto a relatively low voltage at a time t_(L), the OUT signal switcheslogic levels a total of five times due to the noise of the IN signal.

[0005] Although system designers have attempted to reduce noise onsystem busses, and device designers have attempted to reducesusceptibility to input signal noise, the issue is nevertheless becomingmore significant as the voltage levels of input signals continue todecrease. Therefore, there is a need for an input stage having improvedinput signal noise margin and having less susceptibility to inadvertentswitching due to the input signal noise.

SUMMARY OF THE INVENTION

[0006] The present invention is directed to an input stage and methodhaving increased input signal noise margin for generating an outputsignal having a predetermined logic level in response to receiving aninput signal having a voltage level. The input stage includes an inputbuffer that includes an input to which the input signal is applied andan output at which the output signal is provided. The input buffer alsoincludes a reference terminal to which a reference voltage signal isapplied. The input buffer generates an output signal having a logiclevel based on the voltage of the input signal relative to the voltageof the reference voltage signal applied to the reference terminal. Theinput stage further includes a voltage generator that generates avariable output voltage signal that is used as the reference voltage forthe input buffer. The voltage of the output voltage signal provided bythe voltage generator is dependent on the logic value of the outputsignal of the input buffer. In this manner, the reference voltageapplied to the input buffer can be adjusted based on the logic level ofthe output signal in order to provide increased input signal noisemargin.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a signal diagram illustrating various signals of aconventional input buffer.

[0008]FIG. 2 is a signal diagram illustrating various signals of aninput stage according to an embodiment of the present invention.

[0009]FIG. 3 is a functional block diagram illustrating an input stageaccording to another embodiment of the present invention.

[0010]FIG. 4 is a functional block diagram of an input stage accordingto an embodiment of the present invention.

[0011]FIGS. 5a and 5 b are signal diagrams showing various signals of aninput stage according to an embodiment of the present invention.

[0012]FIG. 6 is a block diagram of a memory device including an inputstage according to an embodiment of the present invention.

[0013]FIG. 7 is a block diagram of a computer system including a memorydevice of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

[0014] Embodiments of the present invention are directed to an inputstage having input buffer circuitry using a variable reference voltageto improve input signal noise margin. Certain details are set forthbelow to provide a sufficient understanding of the invention. However,it will be clear to one skilled in the art that the invention may bepracticed without these particular details. In other instances,well-known circuits, control signals, and timing protocols have not beenshown in detail in order to avoid unnecessarily obscuring the invention.

[0015]FIG. 2 is a signal diagram illustrating various signals of aninput stage according to embodiments of the present invention. It willbe appreciated that FIG. 2 is not drawn to scale. As with FIG. 1, whichillustrated various signals of the prior art input buffer circuitry,FIG. 2 illustrates an input signal IN to the input stage, an outputsignal OUT provided by the input stage in response to the IN signal, anda reference voltage VREF. However, in contrast to FIG. 1, the VREFvoltage illustrated in FIG. 2 is variable. That is, the VREF voltagechanges in accordance with the logic level of the OUT signal. TheVREFSSTL voltage is also shown in FIG. 2 for the purposes of comparison.As shown in FIG. 2, two different voltage levels are used for VREF, ahigh reference voltage VREFH, which is used for determining when togenerate an OUT signal having a HIGH logic level in response to a HIGHIN signal, and a low reference voltage VREFL, which is used fordetermining when to generate an OUT signal having a LOW logic level inresponse to a LOW IN signal. In contrast to FIG. 1, the OUT signalillustrated in FIG. 2 does not fluctuate in response to the noise of theIN signal because of the increased noise margin provided by the use of avariable reference voltage.

[0016]FIG. 3 illustrates a series-stub-terminated logic (SSTL) inputstage 300 according to embodiments of the present invention. A constantreference voltage VREFSSTL is applied to a non-inverting input of anoperational amplifier 302, as known in the art. A resistive voltagedivider circuit 306, which includes resistors 308 a-308 d, is coupled tothe output of the op-amp 302. The voltage at a node between the secondand third resistors 308 b and 308 c, respectively, is provided back tothe inverting input of the op-amp 302. As a result, the op-am 302 willgenerate an output signal having a voltage such that the voltageprovided back to the inverting input is approximately equal to VREFSSTL.Consequently, the voltage at the output of the op-amp 302 is greaterthan the VREFSSTL voltage. As shown in FIG. 3, a high reference voltageVREFH is generated at a node 310 located between the first and secondresistors 308 a and 308 b , respectively, and is provided to amultiplexer 314. A low reference voltage VREFL is also generated at anode 312 located between the third and fourth resistors 308 c and 308 d,respectively, and is provided to a multiplexer 316. Both multiplexers314 and 316 can be of conventional design known in the art. As mentionedpreviously, the VREFH voltage is relatively higher than the VREFSSTLvoltage, and the VREFL voltage is relatively lower than the VREFSSTLvoltage. It will be appreciated that the VREFH and VREFL voltages can beadjusted relative to one another by changing the relative resistancevalues of the resistors 308 a-308 d. In an embodiment of the input stage300, one or more of the resistors 308 a-308 d consists of a variableresistor, as well known in the art, to allow for the VREFH and VREFLvoltages to be adjusted.

[0017] The VREFSSTL voltage is also applied to a terminal of a capacitor320 a that is coupled to the node 310 and a terminal of a capacitor 320b that is coupled to the node 312 in order to respond to fluctuations inthe VREFSSTL voltage. That is, by coupling the VREFSSTL voltage to nodes310 and 312 through capacitors 320 a and 320 b, respectively, thevoltage at the respective nodes will maintain the relative relationshipwith the VREFSSTL voltage. Thus, the voltages of VREFH and VREFL withrespect to the VREFSSTL voltage will be held relatively constant. Thecapacitors 320 a and 320 b additionally smooth fluctuations in the VREFHand VREFL voltages.

[0018] The multiplexer 314 provides the VREFH voltage to a VREF input ofan SSTL input buffer 330 in response to an active selection signalSELECT VREFH. Similarly, the multiplexer 316 provides the VREFL voltageto the VREF input of the SSTL input buffer 330 in response to an activeselection signal SELECT VREFL. An external input signal XQS is appliedto an IN input of the input buffer 330. The input buffer 330, which canbe of a conventional design known in the art, compares the voltage ofthe input signal to the reference voltage applied to the VREF input, andgenerates an output signal having an appropriate logic level at anoutput terminal. The output value QS generated by the input buffer 330is also provided to a selection circuit 334 for generating the SELECTVREFH and SELECT VREFL signals for the multiplexers 314 and 316. As willbe explained in more detail below, the selection circuit 334 provides anactive selection signal to the appropriate multiplexer in response tothe logic level of the QS signal. Consequently, the reference voltageapplied to the VREF input to the input buffer 330 will be selected basedon the logic level of the output signal.

[0019] As will be explained in greater detail below, embodiments of thepresent invention use a variable voltage as the reference voltage forthe input buffer 330, the voltage of which is a function of the logicstate of the input buffer. More specifically, if the output signal ofthe input buffer 330 has a HIGH logic level, then a reference voltagehaving the VREFL voltage, where VREFL<VREFSSTL, is provided to the inputbuffer 330 as VREF. Where the output signal of the input buffer 330 is aLOW logic level, then a reference voltage having the VREFH voltage,where VREFH>VREFSSTL, is provided to the input buffer 330 as VREF. Withthis arrangement, the input stage will not switch as soon as the inputcrosses VREFSSTL. Once the input stage generates a HIGH logic level, thereference voltage switches, and the voltage of the input signal willneed to be lower than VREFL for a LOW output signal to be generated.Once the input stage generates a LOW logic level, the reference voltageswitches, and the voltage of the input signal will need to be greaterthan VREFH for a HIGH output signal to be generated. In effect, inputstages according to embodiments of the present invention have a built inhysteresis by using a variable VREF, consequently, resulting in improvednoise immunity.

[0020]FIG. 4 is a schematic drawing of a portion of the input stage 300illustrated in FIG. 3. Operational amplifier block 402 represents theop-amp 302 and the voltage divider circuit 306 (FIG. 3). Transfer gates414 and 416 represent the multiplexers 314 and 316, respectively. Asillustrated in FIG. 4, coupled to the output of the input buffer 330 area transfer gate 404 and a pair of inverters 406 and 410. The controlterminals of the transfer gate 404 are tied to a respective referencevoltage to couple the output of the input buffer 330 to the input of aninverter 406 The output of the inverter 406 and an inverter 412 areprovided to control terminals of the transfer gates 414 and 416 toselect between the VREFH and VREFL as the VREF input of the input buffer330. Specifically, where the output signal of the input buffer 330 has aHIGH logic level, the transfer gate 416 is activated to couple the node312 to the VREF input to provide the VREFL voltage as the referencevoltage, and where the output signal of the input buffer 330 has a LOWlogic level, the transfer gate 414 is activated to couple the node 310to the VREF input to provide the VREFH voltage.

[0021] Operation of the input stage 300 will be described with respectto FIGS. 5a and 5 b. FIG. 5a illustrates the output signal QS inresponse to an input signal XQS. The signal applied to the VREF input ofthe input buffer 330 (FIG. 3) is represented by the signal VREFVAR. Forthe purposes of comparison, FIG. 5a also illustrates the output signalQS_REF generated by an input buffer having a constant reference voltageapplied to its VREF input. The constant reference voltage is representedin FIG. 5a as VREFSSTL.

[0022] At a time t₀, the XQS signal makes a transition from a relativelylow voltage level to a relatively high voltage level. With respect tothe receiving a constant VREFSSTL reference signal, at a time t₁ the XQSsignal exceeds the VREFSSTL voltage level, and as a result, the outputsignal QS_REF switches to a HIGH logic level at a time t₃. For the inputbuffer 330 having the VREFVAR signal applied to its VREF input, the XQSsignal exceeds the VREFVAR signal at a time t₂, which in turn causes theinput buffer 330 to output a HIGH QS signal at a time t₄. As previouslydiscussed, in response to the QS signal going HIGH, the VREFVAR signalfalls to a VREFL voltage level shortly after time t₄ in order to provideimproves noise margin with respect to the XQS signal.

[0023] At a time t₅, the XQS signal makes a transition from a relativelyhigh voltage level to a relatively low voltage level. The voltage of theXQS signal falls below the VREFSSTL voltage level at a time t₆, and inresponse the QS_REF signal switches from a HIGH logic level to a LOWlogic level at a time t₈. At a time t₇, the voltage of the XQS signalfalls below the VREFL voltage, causing the input buffer 330 to force theQS signal from HIGH to LOW at a time t₉. In response to the transitionin the QS signal, the VREFVAR signal switches from the VREFL voltage toa VREFH voltage to provide increased noise margin for the XQS signal.

[0024] As illustrated by FIG. 5a, the input buffer 330 which receives avariable reference voltage applied to its VREF input produces the sameoutput as the conventional input buffer using a constant referencevoltage, but provides improved noise margin for the input signal XQS.The noise margin for the input buffer with a variable reference voltageis represented in FIG. 5a as N_(mH,VREFVAR) and N_(mL,VREFVAR). Comparedwith the noise margin for the input buffer with the constant referencevoltage, that is, N_(mH,VREFssTL) and N_(mL,VREFSSTL), the noise marginsof N_(mH,VREFVAR) and N_(mL,VREFVAR) are clearly greater. It willappreciated that although the difference in the voltage values of VREFLand VREFH relative to the VREFSSTL signal are illustrated in FIG. 5a asbeing approximately equal, the voltages of VREFL and VREFH may betailored, as previously described, such that the differences are unequalif so desired. That is, the VREFL and VREFH voltage levels can becontrolled independently to meet specific design requirements.

[0025] A time delay Δt_(r) and Δt_(f) between the output signals of theinput buffer receiving the constant reference voltage VREFVAR, and theinput buffer stage receiving the variable reference voltage VREFSSTL.The time delays Δt_(r) and Δt_(f) result from the difference in thereference voltage applied to an input buffer. That is, it takes moretime for an input signal to exceed the threshold for the variablereference voltage, either VREFL or VREFH, than for the constantreference voltage VREFSSTL. The time delay will be affected by the slewrate of the input signal XQS. It will be appreciated that the timedelays are minimal and the additional noise margin afforded by thevariable reference voltage is a benefit that may outweigh any adverseaffect on device performance.

[0026]FIG. 5b illustrates the advantage provided by embodiments of thepresent invention over conventional input stages. In FIG. 5b, the inputsignal XQS simulates a “noisy” input signal. That is, at a time to, theXQS signal makes a transition from a relatively low voltage level to arelatively high voltage level. Input signal noise is simulated bydecreasing the voltage of the XQS signal ΔV_(H) at a time t₂. Thevoltage level is maintained until a time t₅, at which time the XQSsignal makes a transition to a relatively low voltage level. At a timet₈, input signal noise is simulated by increasing the voltage of the XQSsignal ΔV_(L). The noise level is maintained until a time t₁, where theXQS signal makes a transition back to a relatively high voltage level.

[0027] With respect to the output signals, QS_REF represents the outputsignal of an input buffer receiving a constant reference voltageVREFSSTL, and QS represents the output signal of an input bufferreceiving a variable reference voltage VREF. Approximately at a time t₁,QS_REF and QS make a transition to a HIGH logic level in response to theXQS signal exceeding the voltages VREFSSTL and VREF, respectively. Thetransition to the HIGH logic level of the QS signal causes the VREFVARsignal to change to a VREFL voltage, which as a result, increases inputsignal noise margin. As mentioned previously, input signal noise issimulated at a time t₂ by decreasing the voltage of the XQS signalΔV_(H). The change in the voltage ΔV_(H) results in the XQS signaldropping below the VREFSSTL voltage at a time t₃. In response, theQS_REF signal makes a transition to a LOW logic level at a time t₄,although the change in voltage ΔV_(H) merely represents input signalnoise. In contrast, because the variable reference voltage VREFVAR isset to the VREFL voltage, providing increased noise margin, the QSsignal is unaffected by the introduction of noise to the XQS signal at atime t₂. The QS signal does not transition to a LOW logic level until atime t₇, which is in response to switching to a relatively low voltagelevel at time t₅ and the voltage of the XQS signal exceeding the VREFLvoltage at a time t₆. As illustrated by the present example, the use ofa variable reference voltage VREFVAR as the reference voltage for aninput buffer can provide additional input noise margin that reducessusceptibility to inadvertent switching due to input signal noise.

[0028] Following the transition of the QS signal at time t₇ from a HIGHlogic level to a LOW logic level, the VREFVAR voltage is adjusted to theVREFH voltage to provide additional noise margin for the XQS signal. Aspreviously described, at a time t₈, the voltage of the XQS is increasedby a ΔV_(L) voltage to simulate input signal noise, although the changein voltage ΔV_(L) merely represents input signal noise. The resultingXQS signal exceeds the constant reference voltage VREFSSTL at a time t₉,and consequently, the QS_REF signal makes a transition from a LOW logiclevel to a HIGH logic level at a time t₁₀. The QS signal, on the otherhand, remains at the LOW logic level despite the input noise ΔV_(L), anddoes not make a transition to a HIGH logic level until a time t₁₃, whichis in response to the XQS signal making a transition at a time t₁₁ andhaving a voltage exceeding the VREFH voltage at a time t₁₂. As with theprevious example where noise was simulated for an input signal having aHIGH logic level, the QS signal was resistant to fluctuations for a LOWXQS signal having a noise of ΔV_(L).

[0029] The present example more clearly demonstrates the advantagesprovided by using a variable reference voltage as the reference voltagefor an input driver in order to provide increased input noise margin.

[0030]FIG. 6 is a block diagram of a dynamic random access memory(“DRAM”) 600 that includes an address decoder 602, control circuit 604,and read/write circuitry 606. The address decoder 602, control circuit604, and read/write circuitry 606 are all coupled to a memory-cell array608. In addition, the address decoder 602 is coupled to an address bus,the control circuit 604 is coupled to a control bus, and the read/writecircuit 606 is coupled to a data bus. In operation, external circuitry,such as a processor or memory controller, applies address, data, andcontrol signals on the respective busses to transfer data to and fromthe DRAM 600. As illustrated in FIG. 6, input stages 620 according toembodiments of the present invention are included in the address decoder602, the control circuit 604, and the read/write circuitry 606. Theinput stages 620 receive input signals and, based on the voltage levelsof the input signals, generate output signals having the appropriatelogic levels for the respective circuitry. It will be appreciated thatalthough FIG. 6 shows an input stage 620 included in the address decoder602, the control circuit 604, and the read/write circuitry 606, theinput stage 620 can be included in greater or fewer of the functionalcircuit blocks without deviating from the scope of the presentinvention. Moreover, the input stage 620 could also be included in othermemory cell circuit blocks that are well known, where inclusion of aninput stage is appropriate.

[0031]FIG. 7 is a block diagram of a computer system 700 includingcomputing circuitry 702. The computing circuitry 702 contains a memory701 that includes input stage circuitry according to embodiments of thepresent invention. The computing circuitry 702 performs variouscomputing functions, such as executing specific software to performspecific calculations or tasks. In addition, the computer system 700includes one or more input devices 704, such as a keyboard or a mouse,coupled to the computer circuitry 702 to allow an operator to interfacewith the computer system. Typically, the computer system 700 alsoincludes one or more output devices 706 coupled to the computercircuitry 702, such output devices typically being a printer or a videoterminal. One or more data storage devices 708 are also typicallycoupled to the computer circuitry 702 to store data or retrieve datafrom external storage media (not shown). Examples of typical storagedevices 708 include hard and floppy disks, tape cassettes, and compactdisc read-only memories (CD-ROMs). The computer circuitry 702 istypically coupled to the memory device 701 through appropriate address,data, and control busses to provide for writing data to and reading datafrom the memory device.

[0032] From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. An input stage for providing an output signal having a logic valuebased on the voltage of an input signal, the input stage comprising: aninput buffer having an input terminal to which the input signal isapplied, an output terminal at which the output signal is provided, anda reference terminal to which a reference voltage signal is applied, theinput buffer generating an output signal having a logic value based onthe voltage of the input signal relative to the voltage of the referencevoltage signal; and a voltage generator having an output terminalcoupled to the reference terminal and further having a control terminalcoupled to the output of the input buffer, the voltage generatorgenerating as the reference voltage signal an output signal having avoltage dependence on the logic value of the output signal of the inputbuffer.
 2. The input stage of claim 1 wherein the voltage generatorcomprises an amplifier having an input to which a initial referencevoltage signal is applied and further having first and second outputterminals, the amplifier generating from the initial reference voltagesignal a high reference voltage signal at the first output terminal anda low reference voltage signal at the second output terminal.
 3. Theinput stage of claim 2 wherein the amplifier comprises: an operationalamplifier having a first input at which the initial reference voltage isapplied, a second input, and an output at which an output signal isprovided; and a voltage divider circuit coupled between the output ofthe operational amplifier and a reference voltage supply, the voltagedivider circuit having a first node at which the high reference voltageis provided, a second node coupled to the second input of theoperational amplifier, and a third node at which the low referencevoltage is provided.
 4. The input stage of claim 2, further comprising amultiplexer having a selection terminal coupled to the output of theinput buffer, an output coupled to the reference terminal, and first andsecond input terminals coupled to the first and second output terminalsof the amplifier, respectively, the multiplexer providing the highreference voltage signal as the reference voltage signal in response tothe output signal of the input buffer having a low logic value and thelow reference voltage signal as the reference voltage signal in responseto the output signal of the input buffer having a high logic value. 5.The input stage of claim 2, further comprising first and second transfergates coupled to the first and second output terminals of the amplifier,respectively, each transfer gate having control terminals to which theoutput of the input buffer is coupled, the first transfer gate couplingthe high reference voltage signal to the reference terminal in responseto the output signal of the input buffer having a low logic value andthe second transfer gate coupling the low reference voltage signal tothe reference terminal in response to the output signal of the inputbuffer having a high logic value.
 6. The input stage of claim 2, furthercomprising a first capacitor having a first terminal coupled to thefirst output terminal of the amplifier and a second capacitor having afirst terminal coupled to the second output terminal of the amplifier,the first and second capacitors each having a second terminal to whichthe initial reference voltage signal is applied.
 7. An input stage forproviding an output signal having a logic value based on the voltage ofan input signal, the input stage comprising: an input buffer having aninput terminal to which the input signal is applied, an output terminalat which the output signal is provided, and a reference terminal towhich a reference voltage signal is applied, the input buffer generatingan output signal having a logic value based on the voltage of the inputsignal relative to the voltage of the reference voltage signal; anoperational amplifier having a first input at which an initial referencevoltage is applied, a second input, and an output; a voltage dividercircuit coupled between the output of the operational amplifier and areference voltage supply, the voltage divider circuit having a firstnode at which a high reference voltage is provided, a second nodecoupled to the second input of the operational amplifier, and a thirdnode at which a low reference voltage is provided; and first and secondtransfer gates coupled to the first and second nodes of the voltagedivider, respectively, each transfer gate having control terminals towhich the output of the input buffer is coupled, the first transfer gatecoupling the high reference voltage signal to the reference terminal inresponse to the output signal of the input buffer having a low logicvalue and the second transfer gate coupling the low reference voltagesignal to the reference terminal in response to the output signal of theinput buffer having a high logic value.
 8. The input stage of claim 7,further comprising a first capacitor having a first terminal coupled tothe first node and a second capacitor having a first terminal coupled tothe third node, the first and second capacitors each having a secondterminal to which the initial reference voltage signal is applied. 9.The input stage of claim 7 wherein the voltage divider comprises: afirst resistor electrically coupled between the output of theoperational amplifier and the first node; a second resistor electricallycoupled between the first node and the second node; a third resistorelectrically coupled between the second node and the third node; and afourth resistor electrically coupled between the third node and thereference voltage supply.
 10. The input stage of claim 7, furthercomprising first and second transfer gates coupled to the first andsecond output terminals of the amplifier, respectively, each transfergate having control terminals to which the output of the input buffer iscoupled, the first transfer gate coupling the high reference voltagesignal to the reference terminal in response to the output signal of theinput buffer having a low logic value and the second transfer gatecoupling the low reference voltage signal to the reference terminal inresponse to the output signal of the input buffer having a high logicvalue.
 11. An input stage including an SSTL input buffer having inputsignal terminal, an output terminal and a reference voltage terminal,the input stage comprising: an amplifier having an input to which ainitial reference voltage signal is applied and further having first andsecond output terminals, the amplifier generating from the initialreference voltage signal a high reference voltage signal at the firstoutput terminal and a low reference voltage signal at the second outputterminal; and a selection circuit having first and second inputterminals coupled to the first and second output terminals of theamplifier, respectively, an output terminal coupled to the referencevoltage terminal, and a control terminal coupled to the output terminalof the input buffer, the selection circuit selectively coupling thefirst and second input terminals to the output terminal based on a logicvalue of an output signal of the input buffer to provide either the highor low reference voltage signals to the input buffer as the referencevoltage signal.
 12. The input stage of claim 11 wherein the amplifiercomprises: an operational amplifier having a first input at which theinitial reference voltage is applied, a second input, and an output atwhich an output signal is provided; and a voltage divider circuitcoupled between the output of the operational amplifier and a referencevoltage supply, the voltage divider circuit having a first node at whichthe high reference voltage signal is provided, a second node coupled tothe second input of the operational amplifier, and a third node at whichthe low reference voltage signal is provided.
 13. The input stage ofclaim 12 wherein the voltage divider comprises: a first resistorelectrically coupled between the output of the operational amplifier andthe first node; a second resistor electrically coupled between the firstnode and the second node; a third resistor electrically coupled betweenthe second node and the third node; and a fourth resistor electricallycoupled between the third node and the reference voltage supply.
 14. Theinput stage of claim 11 wherein the selection circuit comprises amultiplexer having a selection terminal coupled to the output terminalof the input buffer, an output coupled to the reference terminal, andfirst and second input terminals coupled to the first and second outputterminals of the amplifier, respectively, the multiplexer providing thehigh reference voltage signal as the reference voltage signal inresponse to the output signal of the input buffer having a low logicvalue and the low reference voltage signal as the reference voltagesignal in response to the output signal of the input buffer having ahigh logic value.
 15. The input stage of claim 11 wherein the selectioncircuit comprises first and second transfer gates coupled to the firstand second output terminals of the amplifier, respectively, eachtransfer gate having control terminals to which the output of the inputbuffer is coupled, the first transfer gate coupling the high referencevoltage signal to the reference terminal in response to the outputsignal of the input buffer having a low logic value and the secondtransfer gate coupling the low reference voltage signal to the referenceterminal in response to the output signal of the input buffer having ahigh logic value.
 16. The input stage of claim 11, further comprising afirst capacitor having a first terminal coupled to the first node and asecond capacitor having a first terminal coupled to the third node, thefirst and second capacitors each having a second terminal to which theinitial reference voltage signal is applied.
 17. An input stage forproviding an output signal having a logic value based on the voltage ofan input signal, the input stage comprising: an input buffer having aninput terminal to which the input signal is applied, an output terminalat which the output signal is provided, and a reference terminal towhich a reference voltage signal is applied, the input buffer generatingan output signal having a logic value based on the voltage of the inputsignal relative to the voltage of the reference voltage signal; andfirst and second voltage supplies providing first and second voltagelevels, respectively; and a selection circuit having first and secondinput terminals coupled to the first and second voltage supplies,respectively, an output terminal coupled to the reference terminal, anda control terminal coupled to the output terminal of the input buffer,the selection circuit selectively coupling the first and second inputterminals to the output terminal based on the logic value of the outputsignal of the input buffer to provide either the first or second voltagelevels as the reference voltage signal.
 18. The input stage of claim 17wherein the first and second voltage supplies comprise an amplifierhaving an input to which a initial reference voltage signal is appliedand further having first and second output terminals, the amplifiergenerating from the initial reference voltage signal the first voltagelevel at the first output terminal and the second voltage level at thesecond output terminal.
 19. The input stage of claim 18 wherein theamplifier comprises: an operational amplifier having a first input atwhich the initial reference voltage is applied, a second input, and anoutput at which an output signal is provided; and a voltage dividercircuit coupled between the output of the operational amplifier and areference voltage supply, the voltage divider circuit having a firstnode at which the first voltage level is provided, a second node coupledto the second input of the operational amplifier, and a third node atwhich the second voltage level is provided.
 20. The input stage of claim17 wherein the selection circuit comprises a two-input multiplexer. 21.The input stage of claim 17 wherein the selection circuit comprisesfirst and second transfer, each gate having an input terminal coupled toa respective voltage supply and further having control terminals towhich the output of the input buffer is coupled, the first transfer gatecoupling the first voltage level to the reference terminal in responseto the output signal of the input buffer having a low logic value andthe second transfer gate coupling the second voltage level to thereference terminal in response to the output signal of the input bufferhaving a high logic value.
 22. A memory device, comprising: an addressbus; a control bus; a data bus; an address decoder coupled to theaddress bus; a read/write circuit coupled to the data bus; a memory-cellarray coupled to the address decoder, control circuit, and read/writecircuit; and an input stage for providing an output signal having alogic value based on the voltage of an input signal, the input stagecomprising: an input buffer having an input terminal to which the inputsignal is applied, an output terminal at which the output signal isprovided, and a reference terminal to which a reference voltage signalis applied, the input buffer generating an output signal having a logicvalue based on the voltage of the input signal relative to the voltageof the reference voltage signal; and a voltage generator having anoutput terminal coupled to the reference terminal and further having acontrol terminal coupled to the output of the input buffer, the voltagegenerator generating as the reference voltage signal an output signalhaving a voltage dependence on the logic value of the output signal ofthe input buffer.
 23. The memory device of claim 22 wherein the voltagegenerator of the input stage comprises an amplifier having an input towhich a initial reference voltage signal is applied and further havingfirst and second output terminals, the amplifier generating from theinitial reference voltage signal a high reference voltage signal at thefirst output terminal and a low reference voltage signal at the secondoutput terminal.
 24. The memory device of claim 23 wherein the amplifierof the input stage comprises: an operational amplifier having a firstinput at which the initial reference voltage is applied, a second input,and an output at which an output signal is provided; and a voltagedivider circuit coupled between the output of the operational amplifierand a reference voltage supply, the voltage divider circuit having afirst node at which the high reference voltage is provided, a secondnode coupled to the second input of the operational amplifier, and athird node at which the low reference voltage is provided.
 25. Thememory device of claim 23 wherein the input stage further comprises amultiplexer having a selection terminal coupled to the output of theinput buffer, an output coupled to the reference terminal, and first andsecond input terminals coupled to the first and second output terminalsof the amplifier, respectively, the multiplexer providing the highreference voltage signal as the reference voltage signal in response tothe output signal of the input buffer having a low logic value and thelow reference voltage signal as the reference voltage signal in responseto the output signal of the input buffer having a high logic value. 26.The memory device of claim 23 wherein the input stage further comprisesfirst and second transfer gates coupled to the first and second outputterminals of the amplifier, respectively, each transfer gate havingcontrol terminals to which the output of the input buffer is coupled,the first transfer gate coupling the high reference voltage signal tothe reference terminal in response to the output signal of the inputbuffer having a low logic value and the second transfer gate couplingthe low reference voltage signal to the reference terminal in responseto the output signal of the input buffer having a high logic value. 27.The memory device of claim 23 wherein input stage further comprises afirst capacitor having a first terminal coupled to the first outputterminal of the amplifier and a second capacitor having a first terminalcoupled to the second output terminal of the amplifier, the first andsecond capacitors each having a second terminal to which the initialreference voltage signal is applied.
 28. A computer system, comprising:a data input device; a data output device; a processor coupled to thedata input and output devices; and a memory device coupled to theprocessor, the memory device comprising: an address bus; a control bus;a data bus; an address decoder coupled to the address bus; a read/writecircuit coupled to the data bus; a memory-cell array coupled to theaddress decoder, control circuit, and read/write circuit; and an inputstage for providing an output signal having a logic value based on thevoltage of an input signal, the input stage comprising: an input bufferhaving an input terminal to which the input signal is applied, an outputterminal at which the output signal is provided, and a referenceterminal to which a reference voltage signal is applied, the inputbuffer generating an output signal having a logic value based on thevoltage of the input signal relative to the voltage of the referencevoltage signal; and a voltage generator having an output terminalcoupled to the reference terminal and further having a control terminalcoupled to the output of the input buffer, the voltage generatorgenerating as the reference voltage signal an output signal having avoltage dependence on the logic value of the output signal of the inputbuffer.
 29. The computer system of claim 28 wherein the voltagegenerator of the input stage comprises an amplifier having an input towhich a initial reference voltage signal is applied and further havingfirst and second output terminals, the amplifier generating from theinitial reference voltage signal a high reference voltage signal at thefirst output terminal and a low reference voltage signal at the secondoutput terminal.
 30. The computer system of claim 29 wherein theamplifier of the input stage comprises: an operational amplifier havinga first input at which the initial reference voltage is applied, asecond input, and an output at which an output signal is provided; and avoltage divider circuit coupled between the output of the operationalamplifier and a reference voltage supply, the voltage divider circuithaving a first node at which the high reference voltage is provided, asecond node coupled to the second input of the operational amplifier,and a third node at which the low reference voltage is provided.
 31. Thecomputer system of claim 29 wherein the input stage further comprises amultiplexer having a selection terminal coupled to the output of theinput buffer, an output coupled to the reference terminal, and first andsecond input terminals coupled to the first and second output terminalsof the amplifier, respectively, the multiplexer providing the highreference voltage signal as the reference voltage signal in response tothe output signal of the input buffer having a low logic value and thelow reference voltage signal as the reference voltage signal in responseto the output signal of the input buffer having a high logic value. 32.The computer system of claim 29 wherein the input stage furthercomprises first and second transfer gates coupled to the first andsecond output terminals of the amplifier, respectively, each transfergate having control terminals to which the output of the input buffer iscoupled, the first transfer gate coupling the high reference voltagesignal to the reference terminal in response to the output signal of theinput buffer having a low logic value and the second transfer gatecoupling the low reference voltage signal to the reference terminal inresponse to the output signal of the input buffer having a high logicvalue.
 33. The computer system of claim 29 wherein input stage furthercomprises a first capacitor having a first terminal coupled to the firstoutput terminal of the amplifier and a second capacitor having a firstterminal coupled to the second output terminal of the amplifier, thefirst and second capacitors each having a second terminal to which theinitial reference voltage signal is applied.
 34. A method for generatingan output signal having a logic level in response to receiving an inputsignal having a voltage level, the method comprising: comparing thevoltage level of the input signal to a first reference voltage when thecurrent output signal has a first logic level; comparing the voltagelevel of the input signal to a second reference voltage when the currentoutput signal has a second logic level; and switching between the firstand second reference voltages in response to the logic level of theoutput signal changing.
 35. The method of claim 34, further comprising:generating from an initial reference voltage the first reference voltagehaving a voltage relatively greater than the voltage of the initialreference voltage; and generating from the initial reference voltage thesecond reference voltage having a voltage relatively lower than thevoltage of the initial reference voltage.
 36. The method of claim 35wherein the first logic level comprises a LOW logic level and the secondlogic level comprises a HIGH logic level.
 37. The method of claim 35wherein switching between the first and second reference voltagescomprises: selecting the relatively lower reference voltage forcomparison in response to the output signal having the HIGH logic level;and selecting the relatively greater reference voltage in response tothe output signal having the first LOW logic level.
 38. A method forgenerating an output signal having a logic level in response toreceiving an input signal having a voltage level, the method comprising:generating a variable reference voltage to which the voltage level ofthe input signal is compared; comparing the voltage level of the inputsignal to the voltage level of the variable reference voltage todetermine the logic level of the output signal; and adjusting thevoltage level of the variable reference voltage in response to the logiclevel of the output signal changing.
 39. The method of claim 38 whereinadjusting the voltage level of the variable reference voltage comprises:selecting a first reference voltage for comparison in response to theoutput signal having the second logic level; and selecting a secondreference voltage for comparison in response to the output signal havingthe first logic level.
 40. The method of claim 39 wherein the firstvoltage level is a relatively higher voltage than an initial referencevoltage and the second voltage level is a relatively lower voltage thanthe initial reference voltage, the first voltage level is a HIGH logiclevel, and the second voltage level is a LOW logic level.
 41. The methodof claim 38 wherein generating a variable reference voltage comprises:generating from an initial reference voltage a first voltage againstwhich the voltage level of the input signal is compared, the voltage ofthe first voltage relatively greater than the voltage of the initialreference voltage; and generating from the initial reference voltage asecond voltage against which the voltage level of the input signal iscompared, the voltage of the second voltage relatively lower than thevoltage of the initial reference voltage.
 42. The method of claim 38wherein comparing the voltage level of the input signal comprises: whenthe voltage level of the input signal is greater than a first voltagelevel, generating an output signal having a first logic level; and whenthe voltage level of the input signal is less than a second voltagelevel, generating an output signal having a second logic level, thesecond voltage level unequal to the first voltage level.
 43. A methodfor generating an output signal having a logic level in response toreceiving an input signal having a voltage level, the method comprising:when the voltage level of the input signal is greater than a firstvoltage level, generating an output signal having a first logic level;when the voltage level of the input signal is less than a second voltagelevel, generating an output signal having a second logic level, thesecond voltage level unequal to the first voltage level; and switchingbetween the first and second voltage levels when the logic level of theoutput signal changes.
 44. The method of claim 43 wherein switchingbetween the first and second reference voltages comprises: selecting thefirst reference voltage in response to the output signal having thesecond logic level; and selecting the second reference voltage inresponse to the output signal having the first logic level.
 45. Themethod of claim 43 wherein the first voltage level is a relativelyhigher voltage than an initial reference voltage and the second voltagelevel is a relatively lower voltage than the initial reference voltage,the first voltage level is a HIGH logic level, and the second voltagelevel is a LOW logic level.
 46. The method of claim 43, furthercomprising: generating from an initial reference voltage the firstvoltage level having a voltage relatively greater than the voltage ofthe initial reference voltage; and generating the second voltage levelhaving a voltage relatively lower than the voltage of the initialreference voltage.